Kinetics and atomic structure of O adsorption on W(110) from time- and state-resolved photoelectron spectroscopy and full-solid-angle photoelectron diffraction

We have studied the kinetics of the low-pressure adsorption of oxygen on W( 110) via time- and chemical-state-resolved photoelectron spectroscopy (PS) and diffraction (PD). Using high-brightness third-generation synchrotron ra diation from the Advanced Light Source, together with a new photoelectron s pectrometer/diffractometer system, we are able to resolve four distinct che mical states in W 4f spectra (clean surface, bulk, W bound to two O atoms = O2, and W bound to three O atoms = O3) and to measure such spectra in abou t 20 s each so as to follow the kinetics of oxygen adsorption at 3 x 10(-9) Torr from the clean surface to near saturation. The time-dependent transfo rmations from one state of the surface W atoms to another have been determi ned at three temperatures of 298, 360, and 593 K. We also find that, for th is adsorption pressure on our surface, no long-range-ordered structures are observable in LEED, even though the previously observed ordered structures of p(2 x 1), p(2 x 2), and (1 x 1) x 12 are formed at higher pressures of approximately 10(-6) Torr. The room-temperature state-resolved PS data are modelled using a simple Monte Carlo approach which assumes no mobility afte r molecular dissociation, and these calculations are found to describe the experimental data very well. Combining experiment and theory also permits d eriving the sticking coefficient as a function of time, yielding results wh ich agree with prior work. Full-solid-angle PD patterns have also been dete rmined at the end of 298 K oxygen exposure for the O2 and O3 W atoms, and t hese have been analyzed using multiple scattering theory and R-factor analy sis. The final local pseudo-threefold hollow geometries for oxygen are foun d to be very similar to those for a saturated one-monolayer structure of O on W(110) (the (1 x 1) x 12 structure), including a lateral shift of O away from the position corresponding to three equal bond distances, but with so me contraction of the O-W vertical separation in going from O2 to O3 sugges ted. This study indicates considerable potential of such time- and state-re solved PS and diffraction for investigating surface reaction kinetics and s tructure, particularly for the large number of systems that do not exhibit long-range order and in view of future instrumentation improvements that sh ould lead to much shorter data accumulation times and/or higher ambient pre ssures of measurement. (C) 2000 Elsevier Science B.V. All rights reserved.